Ferroelectric chiral smectic liquid crystal composition and liquid crystal device using same

ABSTRACT

A ferroelectric chiral smectic liquid crystal composition, comprising at least one compound represented by the following formula (I): ##STR1## wherein R 1  denotes a linear or branched alkyl group having 1-18 carbon atoms capable of having a substituent, R 2  denotes a linear or branched alkyl group having 1-12 carbon atoms; X 1  denotes a single bond, ##STR2## and at least one compound represented by the following formula (II): ##STR3## wherein R 3  and R 4  respectively denote a linear or branched alkyl group having 1-18 carbon atoms capable of having a substituent, at least one of R 3  and R 4  being optically active; and X 2  and X 3  respectively denote a single bond, ##STR4##

This application is a continuation of application Ser. No. 07/378,929,filed Jul. 12, 1989, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a liquid crystal composition used in aliquid crystal display device, a liquid crystal-optical shutter, etc.,more particularly to a novel liquid crystal composition with improvedresponsiveness to an electric field and a liquid crystal device usingthe liquid crystal composition.

Hitherto, liquid crystal devices have been used as an electro-opticaldevice in various fields. Most liquid crystal devices which have beenput into practice use TN (twisted nematic) type liquid crystals, asshown in "Voltage-Dependent Optical Activity of a Twisted Nematic LiquidCrystal" by M. Schadt and W. Helfrich "Applied Physics Letters" Vol. 18,No. 4 (Feb. 15, 1971) pp. 127-128.

These devices are based on the dielectric alignment effect of a liquidcrystal and utilize an effect that the average molecular axis directionis directed to a specific direction in response to an applied electricfield because of the dielectric anisotropy of liquid crystal molecules.It is said that the limit of response speed is on the order ofmilli-seconds, which is too slow for many uses. On the other hand, asimple matrix system of driving is most promising for application to alarge-area flat display in view of cost, productivity, etc., incombination. In the simple matrix system, an electrode arrangementwherein scanning electrodes and signal electrodes are arranged in amatrix, and for driving, a multiplex driving scheme is adopted whereinan address signal is sequentially, periodically and selectively appliedto the scanning electrodes and prescribed data signals are selectivelyapplied in parallel to the signal electrodes in synchronism with theaddress signal.

When the above-mentioned TN-type liquid crystal is used in a device ofsuch a driving system, a certain electric field is applied to regionswhere a scanning electrode is selected and signal electrodes are notselected or regions where a scanning electrode is not selected and asignal electrode is selected (which regions are so called "half-selectedpoints"). If the difference between a voltage applied to the selectedpoints and a voltage applied to the half-selected points is sufficientlylarge, and a voltage threshold level required for allowing liquidcrystal molecules to be aligned or oriented perpendicular to an electricfield is set to a value therebetween, display devices normally operate.However, in fact, as the number (N) of scanning lines increases, a time(duty ratio) during which an effective electric field is applied to oneselected point when a whole image area (corresponding to one frame) isscanned decreases with a ratio of 1/N. Accordingly, the larger thenumber of scanning lines are, the smaller is the voltage difference ofan effective value applied to a selected point and non-selected pointswhen scanning is repeatedly effected. As a result, this leads tounavoidable drawbacks of lowering of image contrast or occurrence ofinterference or crosstalk. These phenomena are regarded as essentiallyunavoidable problems appearing when a liquid crystal having nobistability (i.e. liquid crystal molecules are horizontally orientedwith respect to the electrode surface as stable state and is verticallyoriented with respect to the electrode surface only when an electricfield is effectively applied) is driven (i.e. repeatedly scanned) bymaking use of a time storage effect. To overcome these drawbacks, thevoltage averaging method, the two-frequency driving method, the multiplematrix method, etc. has been already proposed. However, any method isnot sufficient to overcome the above-mentioned drawbacks. As a result,it is the present state that the development of large image area or highpackaging density in respect to display elements is delayed because itis difficult to sufficiently increase the number of scanning lines.

To overcome drawbacks with such prior art liquid crystal devices, theuse of liquid crystal devices having bistability has been proposed byClark and Lagerwall (e.g. Japanese Laid-Open Patent Appln. No.56-107216, U.S. Pat. No. 4,367,924, etc.). In this instance, as theliquid crystals having bistability, ferroelectric liquid crystals havingchiral smectic C-phase (SmC*) or H-phase (SmH*) are generally used.These liquid crystals have bistable states of first and second stablestates with respect to an electric field applied thereto. Accordingly,as different from optical modulation devices in which theabove-mentioned TN-type liquid crystals are used, the bistable liquidcrystal molecules are oriented to first and second optically stablestates with respect to one and the other electric field vectors,respectively. Further, this type of liquid crystal has a property(bistability) of assuming either one of the two stable states inresponse to an applied electric and retaining the resultant state in theabsence of an electric field.

In addition to the above-described characteristic of showingbistability, such a ferroelectric liquid crystal (hereinafter sometimesabbreviated as "FLC") has an excellent property, i.e., a high-speedresponsiveness. This is because the spontaneous polarization of theferroelectric liquid crystal and an applied electric field directlyinteract with each other to induce transition of orientation states. Theresultant response speed is faster than the response speed due to theinteraction between dielectric anisotropy and an electric field by 3 to4 digits.

Thus, a ferroelectric liquid crystal potentially has very excellentcharacteristics, and by making use of these properties, it is possibleto provide essential improvements to many of the above-mentionedproblems with the conventional TN-type devices. Particularly, theapplication to a high-speed optical shutter and a display of a highdensity and a large picture is expected.

A simple matrix display apparatus including a device comprising such aferroelectric liquid crystal layer between a pair of substrates may bedriven according to a driving method as disclosed in, e.g., JapaneseLaid-Open Patent Applications Nos. 193426/1984, 193427/1984, 156046/1985and 156047/1985.

FIGS. 4A and 4B are waveform diagrams showing driving voltage waveformsadopted in driving a ferroelectric liquid crystal panel as an embodimentof the liquid crystal device according to the present invention. FIG. 5is a plan view of such a ferroelectric liquid crystal panel 51 having amatrix electrode structure. Referring to FIG. 5, the panel 51 comprisesscanning lines 52 and data lines 53 intersecting with the scanninglines. Each intersection comprises a ferroelectric liquid crystaldisposed between a scanning line 52 and a data line 53 to form a pixel.

Referring to FIG. 4A, at S_(S) is shown a selection scanning signalwaveform applied to a selected scanning line, at S_(N) is shown anon-selection scanning signal waveform applied to a non-selectedscanning line, at I_(S) is shown a selection data signal waveform(providing a black display state) applied to a selected data line, andat I_(N) is shown a non-selection data signal waveform applied to anon-selected data line. Further, at I_(S) -S_(S) and I_(N) -S_(S) in thefigure are shown voltage waveforms applied to pixels on a selectedscanning line, whereby a pixel supplied with the voltage I_(S) -S_(S)assumes a black display state and a pixel supplied with the voltageI_(N) -S_(S) assumes a white display state. FIG. 4B shows a time-serialwaveform used for providing a display state as shown in FIG. 6.

In the driving embodiment shown in FIGS. 4A and 4B, a minimum durationΔt of a single polarity voltage applied to a pixel on a selectedscanning line corresponds to the period of a writing phase t₂, and theperiod of a one-line clearing phase t₁ is set to 2Δt.

The parameters V_(S), V_(I) and Δt in the driving waveforms shown inFIGS. 4A and 4B are determined depending on switching characteristics ofa ferroelectric liquid crystal material used.

FIG. 7 shows a V-T characteristic, i.e., a change in transmittance Twhen a driving voltage denoted by (V_(S) +V_(I)) is changed while a biasratio as mentioned hereinbelow is kept constant. In this embodiment, theparameters are fixed at constant values of Δt=50 μs and a bias ratioV_(I) /(V_(I) +V_(S))=1/3. On the right side of FIG. 7 is shown a resultwhen the voltage (I_(N) -S_(S)) shown in FIG. 4 is applied to a pixelconcerned, and on the left side of FIG. 7 is shown a result when thevoltage (I_(S) -S_(S)) is applied to a pixel concerned, respectivelywhile increasing the voltage (V_(S) +V_(I)). On both sides of theordinate, the absolute value of the voltage (V_(S) +V_(I)) is separatelyindicated. Herein, a voltage V₁ denotes the absolute value of (V_(S)+V_(I)) required for switching from a white state to a black state byapplying a voltage signal V_(B) ² shown in FIG. 4A, a voltage V₂ denotesthe absolute value of (V_(S) +V_(I)) required for switching (resetting)a black state to a white state by applying a voltage V_(R) at I_(N)-S_(S), and a voltage V₃ is the value of (V_(S) +V_(I)) beyond which apixel concerned written in white is unexpectedly inverted into a blackstate. In this instance, a relationship of V₂ <V₁ <V₃ holds. The voltageV₁ may be referred to as a threshold voltage in actual drive and thevoltage V₃ may be referred to as a crosstalk voltage. Such a crosstalkvoltage V₃ is generally present in actual matrix drive of aferroelectric liquid crystal device. In an actual drive, ΔV=(V₃ -V₁)provides a range of |V_(S) +V_(I) | allowing a matrix drive and may bereferred to as a (driving) voltage margin, which is preferably largeenough. It is of course possible to increase the value of V₃ and thus ΔV(=V₃ -V₁) by increasing the bias ratio (i.e., by causing the bias ratioto approach a unity). However, a large bias ratio corresponds to a largeamplitude of a data signal and leads to an increase in flickering and alower contrast, thus being undesirable in respect of image quality.According to our study, a bias ratio of about 1/3-1/4 was practical. Onthe other hand, when the bias ratio is fixed, the voltage margin ΔVstrongly depends on the switching characteristics of a liquid crystalmaterial used, and it is needless to say that a liquid crystal materialproviding a large ΔV is very advantageous for matrix drive.

The upper and lower limits of application voltages and a differencetherebetween (driving voltage margin ΔV) by which selected pixels arewritten in two states of "black" and "white" and non-selected pixels canretain the written "black" and "white" states at a constant temperatureas described above, vary depending on and are inherent to a particularliquid crystal material used. Further, the driving margin is deviatedaccording to a change in environmental temperature, so that optimumdriving voltages should be set in an actual display apparatus accordingto a liquid crystal material used and an environmental temperature.

In a practical use, however, when the display area of a matrix displayapparatus is enlarged, the differences in environmental conditions (suchas temperature and cell gap between opposite electrodes) naturallyincrease, so that it becomes impossible to obtain a good quality ofimage over the entire display area by using a liquid crystal materialhaving a small driving voltage margin.

On the other hand, it is known that the ferroelectric liquid crystalmolecules under such non-helical conditions are disposed in successionso that their directors (longer molecular axes) are gradually twistedbetween the substrates and do not show a uniaxial orientation oralignment (i.e., in a splay alignment state). A problem in this case isa low transmittance through the liquid crystal layer.

Transmitted light intensity I through a liquid crystal is given by thefollowing equation with respect to the incident light intensity I_(O)under cross nicols when the uniaxial alignment of the molecules isassumed:

    I=I.sub.O sin.sup.2 (4θa)·sin.sup.2 (πΔnd/λ) (1),

wherein Δn denotes the refractive index anisotropy of the FLC; d, thecell thickness; λ, the wavelength of the incident light; and θa, a halfof the angle between two stable states (tilt angle).

When a conventional FLC cell is used, it has been experimentally knownthat θa is 5-8 degrees under a twisted alignment condition. The controlof physical properties affecting the term Δndπ/λ cannot be easilyperformed, so that it is desired to increase θa to increase I. However,this has not been successfully accomplished by only a static alignmenttechnique.

With respect to such a problem, it has been proposed to utilize a torquerelating to a dielectric anisotropy Δε of an FLC (1983 SID report fromAT & T; Japanese Laid-Open Patent Applns. 245142/1986, 246722/1986,246723/1986, 246724/1986, 249024/1986 and 249025/1986). Morespecifically, a liquid crystal molecule having a negative Δε tends tobecome parallel to the substrates under application of an electricfield. By utilizing this property, if an effective value of AC electricfield is applied even in a period other than switching, theabove-mentioned twisted alignment is removed, so that θa is increased toprovide an increased transmittance (AC stabilization effect). A torqueΓP_(S) acting on FLC molecules involved in switching of states and atorque ΓΔε acting on FLC molecules relating to the AC stabilizationeffect are respectively proportional to physical properties as shown inthe following formulas:

    ΓP.sub.s ∝P.sub.S ·E                 (2)

    ΓΔε∝1/2Δε·ε.sub.0 ·E.sup.2                                         ( 3)

The above formula (3) apparently shows that the sign and absolute valueof Δε of the FLC play an important role.

FIG. 8 attached hereto shows the change of θa versus Vrms experimentallymeasured for 4 FLCs having different values of Δε. The measurement wasconducted under application of AC rectangular pulses of 60 KHz so as toremove the influence of P_(S). The curves (I)-(IV) correspond to theresults obtained by using FLCs showing the following Δε values

(I) Δε≃-5.5, (II) Δε≃-3.0,

(III) Δε≃-0, (IV) Δε≃1.0.

Qualitatively, the order of Δε was (I)<(II)<(III)<(IV).

As is clear from the graph in FIG. 8, a larger negative value of Δεprovides a large θa at a lower voltage and thus contributes to provisionof an increased I.

The transmittances obtained by using the liquid crystals (I) and (III)were 15% for (I) and 6% for (III) (under application of rectangular ACwaveforms of 60 kHz and ±8 V), thus showing a clear difference.

As is known from the above examples, the display characteristics of anSSFLC (Surface-Stabilized FLC) can be remarkably changed by controllingthe properties relating to Δε and P_(S).

In order to provide a ferroelectric liquid crystal composition having anegatively large Δε, it is most effective to include a compound having anegative Δε with a large absolute value. For example, it is possible toobtain a compound having a negatively large Δε by introducing a halogenor cyano group in a shorter axis direction of a molecule or byintroducing a heterocyclic skeleton in a molecule.

The magnitude of Δε of a compound having a negative Δε substantiallyvaries depending on the structure thereof. Some examples of suchcompounds are shown below: ##STR5## Herein, R and R' respectively denotean alkyl group. These may be classified roughly into three groupsincluding compounds having a negatively small Δε (|Δε|≦2), compoundshaving a negatively medium Δε(2<|Δε|≦10) and compounds having anegatively large Δε(|Δε|>10). Among these, compounds having a |Δε| of ≦2have little effect of increasing |Δε|. Compounds having a |Δε| of >10are very effective in increasing |Δε| but those available heretofore areonly dicyanohydroquinone derivatives.

However, a dicyanohydroquinone derivative, while it has a large|Δε|-increasing effect, has a high viscosity, so that it is liable todegrade a switching characteristic when its content is increased. On theother hand, among the compounds having a medium |Δε| (2<|Δε|≦10), somecompounds have a moderately low viscosity while their |Δε|-increasingeffect is somewhat lower than those having a large |Δε|.

From the above consideration, it is essential to select a compoundhaving a negative anisotropy, preferably one having a |Δε| of >2, andmixing it with an appropriately selected other compound in a properlyselected mixing ratio.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a chiral smectic liquidcrystal composition having a large driving voltage margin adapted forproviding a practical ferroelectric liquid crystal device and a widedriving voltage margin affording satisfactory drive of entire pixelseven when some degree of temperature fluctuation is present over adisplay area comprising the pixels of a liquid crystal device.

Another object of the present invention is to provide a liquid crystalcomposition further containing a mesomorphic compound having a negativedielectric anisotropy to show an AC stabilization effect providingremarkably improved display characteristics.

A further object of the present invention is to provide a liquid crystaldevice using such a liquid crystal composition and showing improveddriving and display characteristics.

According to the present invention, there is provided a ferroelectricchiral smectic liquid crystal composition, comprising:

at least one compound represented by the following formula (I): ##STR6##wherein R₁ denotes a linear or branched alkyl group having 1-18 carbonatoms capable of having a substituent; R₂ denotes a linear or branchedalkyl group having 1-12 carbon atoms; X₁ denotes a single bond, ##STR7##and

at least one compound represented by the following formula (II):##STR8## wherein R₃ and R₄ respectively denote a linear or branchedalkyl group having 1-18 carbon atoms capable of having a substituent; atleast one of R₃ and R₄ being optically active; and X₂ and X₃respectively denote a single bond, ##STR9##

According to the present invention, there is further provided aferroelectric liquid crystal composition as described above furthercomprising a mesomorphic compound having a negative dielectricanisotropy, which is preferably one having a Δε<-2, more preferablyΔε<-5, most preferably Δε<-10.

The present invention further provides a liquid crystal devicecomprising a pair of substrates and such a ferroelectric liquid crystalcomposition as described above disposed between the electrode plates.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a liquid crystal display deviceusing a ferroelectric liquid crystal;

FIGS. 2 and 3 are schematic perspective views of a device cellembodiment for illustrating the operation principle of a ferroelectricliquid crystal device;

FIG. 4A shows unit driving waveforms used in an embodiment of thepresent invention; FIG. 4B is time-serial waveforms comprising asuccession of such unit waveforms;

FIG. 5 is a plan view of a ferroelectric liquid crystal panel having amatrix electrode structure;

FIG. 6 is an illustration of a display pattern obtained by an actualdrive using the time-serial waveforms shown in FIG. 4B;

FIG. 7 is a V-T characteristic chart showing a change in transmittanceunder application of varying drive voltages; and

FIG. 8 is a graph showing changes in tilt angle θa versus effectivevoltage Vrms with respect to several ferroelectric liquid crystalshaving different values of dielectric anisotropy Δε.

DETAILED DESCRIPTION OF THE INVENTION

Preferred examples of the compounds represented by the above-mentionedgeneral formula (I) may include those represented by the followingformulas (I-a) to (I-d). ##STR10##

In the above-formulas (I-a)-(I-d), R₁, R₂ and X₁ are the same as in thegeneral formula (I). Preferred examples of X₁ may include a single bond,##STR11## Further, preferred example of R₁ may include a linear alkylgroup.

In the formula (II), R₃, R₄, X₂ and X₃ are respectively the same as inthe general formula (II). Preferred examples of X₂ and X₃ may includethe following combinations (II-i) to (II-viii):

(II-i) X₂ is a single bond and X₃ is --O--,

(II-ii) X₂ is a single bond and X₃ is ##STR12## (II-iii) X₂ is a singlebond and X₃ is ##STR13## (II-iv) X₂ is a single bond and X₃ is a singlebond, (II-v) X₂ is --O-- and X₃ is --O--,

(II-vi) X₂ is --O-- and X₃ is ##STR14## (II-vii) X₂ is --O-- and X₃ is##STR15## (II-viii) X₂ is --O-- and X₃ is a single bond.

Further, preferred examples of R₃ and R₄ in the formula (II) may includethe following combinations (II-ix) to (II-xi):

(II-ix) R₃ is an n-alkyl group and R₄ is ##STR16## (II-x) R₃ is##STR17## and R₄ is an n-alkyl group, (II-xi) R₃ is ##STR18## and R₄ is##STR19## wherein x and y respectively 0-7, and R₅ and R₆ respectivelydenote a linear or branched alkyl group.

Specific examples of the compounds represented by the above-mentionedgeneral formula (I) may include those shown by the following structuralformulas. ##STR20##

A representative example of synthesis of a compound represented by theformula (I) is described below.

Synthesis Example 1 (Synthesis of compound Example No. 1-4)

(1) A small amount of triethylamine was added to 10 g (53.6 mmol) oftrans-4-n-propylcyclohexanecarbonyl chloride dissolved in 30 ml ofethanol, followed by 10 hours of stirring at room temperature. Thereaction mixture was poured into 100 ml of iced water, acidified with6N-hydrochloric acid and extracted with isopropyl ether. The organiclayer was repeatedly washed with water until the aqueous layer becameneutral, followed by drying with magnesium sulfate, distilling-off ofthe solvent and purification by silica gel column chromatography toobtain 9.9 g of ethyl trans-4-n-propylcyclohexylcarboxylate.

(2) 0.73 g (19.1 mmol) of lithium aluminum hydride was added to 30 ml ofdry ether, heat-refluxed for 1 hour and cooled to 10° C. on an icedwater bath, and a solution of 5 g (25.5 mmol) of ethyltrans-4-n-propylcyclohexylcarboxylate in 30 ml of dry ether wasgradually added dropwise thereto. After the addition, the mixture wasstirred for 1 hour and further heat-refluxed for 1 hour, followed bytreatment with ethyl acetate and with 6N-hydrochloric acid and pouringinto 200 ml of iced water. The resultant mixture was extracted withisopropyl ether and the organic layer was successively washed withwater, sodium hydroxide aqueous solution and water, followed by dryingwith magnesium sulfate, distilling-off of the solvent and purificationby silica gel column chromatography to obtain 3.5 g oftrans-4-n-propylcyclohexylmethanol.

(3) 3.4 g (22.4 mmol) of trans-4-n-propylcyclohexylmethanol wasdissolved in 20 ml of pyridine. A solution of 5.3 g of p-toluenesulfonylchloride in 20 ml of pyridine was added dropwise thereto below 5° C. onan iced water bath under cooling, followed by 10 hours of stirring atroom temperature and pouring into 200 ml of iced water. The resultantmixture was acidified with 6N-hydrochloric acid and extracted withisopropyl ether. The organic layer was repeatedly washed with wateruntil the aqueous layer became neutral, followed by drying withmagnesium sulfate and distilling-off of the solvent, to obtaintrans-4-n-propylcyclohexylmethyl-p-toluenesulfonate.

(4) 6.3 g (20.2 mmol) of 5-decyl-2-(4'-hydroxyphenyl)pyrimidine wasdissolved in 40 ml of dimethylformamide and 1.5 g of potassium hydroxide(85%) was added thereto, followed by 1 hour of stirring at 100° C.Further, 6.9 g of trans-4-n-propylcyclohexylmethyl-p-toluenesulfonatewas added thereto, followed further by 4 hours of stirring at 100° C.After the reaction, the reaction mixture was poured into 200 ml of icedwater and extracted with benzene. The organic layer was washed withwater and dried with magnesium sulfate, followed by distilling-off ofthe solvent, purification by silica gel column chromatography andrecrystallization from ethanol/ethyl acetate mixture solvent to obtainthe above Example compound No. 1-4.

IR (cm⁻¹): 2920, 2840, 1608, 1584, 1428, 1258, 1164, 800

Phase transition temperature (°C.) ##STR21##

Sm2: smectic phase other than SmA and SmC (un-identified)

Specific examples of the compounds represented by the above-mentionedgeneral formula (II) may include those shown by the following structuralformulas. ##STR22##

The compounds represented by the formula (II) may be synthesized throughprocesses as disclosed by, e.g., Japanese Laid-Open Patent Applications(KOKAI) 93170/1986, 24576/1986, 129170/1986, 200972/1986, 200973/1986,215372/1986 and 291574/1986. More specifically, for example, thefollowing reaction scheme may be used for the synthesis. ##STR23## R₃,R₄ and X₃ are the same as defined above.

In a preferred embodiment, the ferroelectric chiral smectic liquidcrystal composition according to the present invention further comprisesa mesomorphic compound having a egative dielectric anisotropy, which ispreferably selected from those represented by the following formulas(III-1) to (III-5): ##STR24## wherein Ra and Rb respectively denote alinear or branched alkyl group capable of having a substituent; Xa andXd respectively denote a single bond, ##STR25## Xb and Xc respectivelydenote a single bond, ##STR26## Aa and Ab respectively denote a singlebond, ##STR27## with proviso that when Aa and Ab are both single bonds,Xb and Xc are both single bonds, and Xa and Xd are both single bonds or--O--, or Xa is ##STR28## and Xd is ##STR29## and Ya and Yb arerespectively cyano group, halogen or hydrogen with proviso that Ya andYb cannot be hydrogen simultaneously; ##STR30## wherein Re and Rfrespectively denote a linear or branched alkyl group capable of having asubstituent; Xe and Xh are respectively a single bond, ##STR31## Xf andXg are respectively ##STR32## os a single bond; and Ae and Af arerespectively ##STR33## or a single bond with proviso that Ae and Afcannot be a single bond simultaneously; ##STR34## wherein Ai is a singlebond or ##STR35## Aj is a single bond, ##STR36## Ri and Rj arerespectively a linear or branched alkyl group capable of having asubstituent with proviso that Ri and Rj are linear alkyl groups when Ajis a single bond; Z₁ is --O-- or --S--; Xi and Xk are respectively asingle bond, ##STR37## Xj is a single bond, ##STR38## --CH₂ O-- or--OCH₂ with proviso that Xi is a single bond when Ai is a single bond,Xj is not a single bond when Aj is ##STR39## and Xk is a single bondwhen Aj is a single bond; ##STR40## wherein Rl and Rm are respectively alinear or branched alkyl group capable of having a substituent; Al andAm are respectively a single bond, ##STR41## with proviso that Al and Amcannot be a single bond simultaneously; Xl is a single bond, ##STR42##and Xm is a single bond, ##STR43## --CH₂ O--, --OCH₂ --, --CH₂ CH₂ -- or--C.tbd.C--; ##STR44## wherein Rn and Ro are respectively a linear orbranched alkyl group capable of having a substituent; Xn and Xq arerespectively a single bond, ##STR45## Xo and Xp are respectively asingle bond, ##STR46## --CH₂ O--, --OCH₂ -- or --CH₂ CH₂ --; An and Apare respectively a single bond, ##STR47## Ao is ##STR48## and Z₂ is##STR49##

In the above formulas (III-1) to (III-5), the alkyl groups Ra-Ro mayrespectively have 1-18 carbon atoms, preferably 4-16 carbon atoms,further preferably 6-12 carbon atoms.

Specific examples of mesomorphic compounds represented by the generalformulas (III-1) to (III-5) may respectively include those denoted bythe structural formulas shown below. ##STR50##

The mesomorphic compound having a negative dielectric anisotropy Δε maypreferably have Δε<-2, preferably Δε<-5, further preferably Δε<-10.

The liquid crystal composition according to the present invention may beobtained by mixing at least one species of the compound represented bythe formula (I), at least one species of the compound represented by theformula (II), optionally at least one species of a mesomorphic compoundhaving a negative dielectric anisotropy and another mesomorphic compoundin appropriate proportions. The liquid crystal composition according tothe present invention may preferably be formulated as a ferroelectricliquid crystal composition, particularly a ferroelectric chiral smecticliquid crystal composition.

Specific examples of another mesomorphic compound as described above mayinclude those denoted by the following structure formulas. ##STR51##

In formulating the liquid crystal composition according to the presentinvention, it is desirable to mix 1-300 wt. parts each, preferably 2-100wt. parts each, of a compound represented by the formula (I) and acompound represented by the formula (II) with 100 wt. parts of anothermesomorphic compound as mentioned above which can be composed of two ormore species.

Further, when two or more species of either one or both of the compoundsrepresented by the formulas (I) and (II) are used, the two or morespecies of the compound of the formula (I) or (II) may be used in atotal amount of 1-500 wt. parts, preferably 2-100 wt. parts, per 100 wt.parts of another mesomorphic compound as described above which can becomposed of two or more species.

Further, the weight ratio of the compound of the formula (I)/thecompound of the formula (II) may desirably be 1/300-300/1, preferably1/50-50/1. When two or more species each of the compounds of theformulas (I) and (II) are used, the weight ratio of the total amount ofthe compounds of the formula (I)/the total amounts of the compounds ofthe formula (II) may desirably be 1/500-500/1, preferably 1/50-50/1.

Further, the total amounts of the compounds of the formulas (I) and (II)may desirably be 2-600 wt. parts, preferably 4-200 wt. parts, when onespecies each is selected from the formulas (I) and (II), or 2-1000 wt.parts, preferably 4-200 wt. parts, when two or more species are selectedfrom at least one of the formulas (I) and (II), respectively, withrespect to 100 wt. parts of the above-mentioned another mesomorphiccompound which may be composed of two or more species.

Further, a mesomorphic compound having a negative dielectric anisotropyas described above can be contained in a proportion of 1-98 wt. % of theliquid crystal composition of the present invention so as to provide acomposition having a negative dielectric anisotropy. Particularly, whena mesomorphic compound having Δε<-2 is used, it may be contained in aproportion of 1-70 wt. %, preferably 1-50 wt. %, of the liquid crystalcomposition of the present invention.

Further, the total of the compounds of the formulas (I) and (II) and themesomorphic compound having a negative dielectric anisotropy canconstitute 3-100 wt. % of the liquid crystal composition of the presentinvention.

The ferroelectric liquid crystal device according to the presentinvention may preferably be prepared by heating the liquid crystalcomposition prepared as described above into an isotropic liquid undervacuum, filling a blank cell comprising a pair of oppositely spacedelectrode plates with the composition, gradually cooling the cell toform a liquid crystal layer and restoring the normal pressure.

FIG. 1 is a schematic sectional view of an embodiment of theferroelectric liquid crystal device prepared as described above forexplanation of the structure thereof.

Referring to FIG. 1, the ferroelectric liquid crystal device includes aferroelectric liquid crystal layer 1 disposed between a pair of glasssubstrates 2 each having thereon a transparent electrode 3 and aninsulating alignment control layer 4. Lead wires 6 are connected to theelectrodes so as to apply a driving voltage to the liquid crystal layer1 from a power supply 7. Outside the substrates 2, a pair of polarizers8 are disposed so as to modulate incident light I₀ from a light source 9in cooperation with the liquid crystal 1 to provide modulated light I.

Each of two glass substrates 2 is coated with a transparent electrode 3comprising a film of In₂ O₃, SnO₂ or ITO (indium-tin-oxide) to form anelectrode plate. Further thereon, an insulating alignment control layer4 is formed by rubbing a film of a polymer such as polyimide with gauzeor acetate fiber-planted cloth so as to align the liquid crystalmolecules in the rubbing direction. Further, it is also possible tocompose the alignment control layer of two layers, e.g., by firstforming an insulating layer of an inorganic material, such as siliconnitride, silicon nitride containing hydrogen, silicon carbide, siliconcarbide containing hydrogen, silicon oxide, boron nitride, boron nitridecontaining hydrogen, cerium oxide, aluminum oxide, zirconium oxide,titanium oxide, or magnesium fluoride, and forming thereon an alignmentcontrol layer of an organic insulating material, such as polyvinylalcohol, polyimide, polyamide-imide, polyester-imide, polyparaxylylene,polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride,polyvinyl acetate, polyamide, polystyrene, cellulose resin, melamineresin, urea resin, acrylic resin, or photoresist resin. Alternatively,it is also possible to use a single layer of inorganic insulatingalignment control layer or organic insulating alignment control layer.An inorganic insulating alignment control layer may be formed by vapordeposition, while an organic insulating alignment control layer may beformed by applying a selection of an organic insulating material or aprecursor thereof in a concentration of 0.1 to 20 wt. %, preferably0.2-10 wt. %, by spinner coating, dip coating, screen printing, spraycoating or roller coating, followed by curing or hardening underprescribed hardening condition (e.g., by heating). The insulatingalignment control layer may have a thickness of ordinarily 30 Å-1micron, preferably 30-3000 Å, further preferably 50-1000 Å. The twoglass substrates 2 with transparent electrodes 3 (which may beinclusively referred to herein as "electrode plates") and further withinsulating alignment control layers 4 thereof are held to have aprescribed (but arbitrary) gap with a spacer 5. For example, such a cellstructure with a prescribed gap may be formed by sandwiching spacers ofsilica beads or alumina beads having a prescribed diameter with twoglass plates, and then sealing the periphery thereof with, e.g., anepoxy adhesive. Alternatively, a polymer film or glass fiber may also beused as a spacer. Between the two glass plates, a ferroelectric liquidcrystal is sealed up to provide a ferroelectric liquid crystal layer 1in a thickness of generally 0.5 to 20 microns, preferably 1 to 5microns.

The ferroelectric liquid crystal provided by the composition of thepresent invention may desirably assume a SmC* phase (chiral smectic Cphase) in a wide temperature range including room temperature(particularly, broad in a lower temperature side) and also shows widedrive voltage margin and drive temperature margin when contained in adevice.

Particularly, in order to show a good alignment characteristic to form auniform monodomain, the ferroelectric liquid crystal may show a phasetransition series comprising isotropic phase--Ch phase (cholestericphase)--SmA phase (smectic A phase)--SmC* phase (chiral smectic C phase)on temperature decrease.

The transparent electrodes 3 are connected to the external power supply7 through the lead wires 6. Further, outside the glass substrates 2,polarizers 8 are applied. The device shown in FIG. 1 is of atransmission type and is provided with a light source 9.

FIG. 2 is a schematic illustration of a ferroelectric liquid crystalcell (device) for explaining operation thereof. Reference numerals 21aand 21b denote substrates (glass plates) on which a transparentelectrode of, e.g., In₂ O₃, SnO₂, ITO (indium-tin-oxide), etc., isdisposed, respectively. A liquid crystal of an SmC*-phase (chiralsmectic C phase) in which liquid crystal molecular layers 22 are alignedperpendicular to surfaces of the glass plates is hermetically disposedtherebetween. Full lines 23 show liquid crystal molecules. Each liquidcrystal molecule 23 has a dipole moment (P⊥) 24 in a directionperpendicular to the axis thereof. The liquid crystal molecules 23continuously form a helical structure in the direction of extension ofthe substrates. When a voltage higher than a certain threshold level isapplied between electrodes formed on the substrates 21a and 21b, ahelical structure of the liquid crystal molecule 23 is unwound orreleased to change the alignment direction of respective liquid crystalmolecules 23 so that the dipole moments (P⊥) 24 are all directed in thedirection of the electric field. The liquid crystal molecules 23 have anelongated shape and show refractive anisotropy between the long axis andthe short axis thereof. Accordingly, it is easily understood that when,for instance, polarizers arranged in a cross nicol relationship, i.e.,with their polarizing directions crossing each other, are disposed onthe upper and the lower surfaces of the glass plates, the liquid crystalcell thus arranged functions as a liquid crystal optical modulationdevice of which optical characteristics vary depending upon the polarityof an applied voltage.

Further, when the liquid crystal cell is made sufficiently thin (e.g.,less than about 10 microns), the helical structure of the liquid crystalmolecules is unwound to provide a non-helical structure even in theabsence of an electric field, whereby the dipole moment assumes eitherof the two states, i.e., Pa in an upper direction 34a or Pb in a lowerdirection 34b as shown in FIG. 3, thus providing a bistable condition.When an electric field Ea or Eb higher than a certain threshold leveland different from each other in polarity as shown in FIG. 3 is appliedto a cell having the above-mentioned characteristics, the dipole momentis directed either in the upper direction 34a or in the lower direction34b depending on the vector of the electric field Ea or Eb. Incorrespondence with this, the liquid crystal molecules are oriented ineither of a first stable state 33a and a second stable state 33b.

When the above-mentioned ferroelectric liquid crystal is used as anoptical modulation element, it is possible to obtain two advantages.First is that the response speed is quite fast. Second is that theorientation of the liquid crystal shows bistability. The secondadvantage will be further explained, e.g., with reference to FIG. 3.When the electric field Ea is applied to the liquid crystal molecules,they are oriented in the first stable state 33a. This state is stablyretained even if the electric field is removed. On the other hand, whenthe electric field Eb of which direction is opposite to that of theelectric field Ea is applied thereto, the liquid crystal molecules areoriented to the second stable state 33b, whereby the directions ofmolecules are changed. This state is similarly stably retained even ifthe electric field is removed. Further, as long as the magnitude of theelectric field Ea or Eb being applied is not above a certain thresholdvalue, the liquid crystal molecules are placed in the respectiveorientation states.

When such a ferroelectric liquid crystal device comprising aferroelectric liquid crystal composition as described above between apair of electrode plates is constituted as a simple matrix displaydevice, the device may be driven by a driving method as disclosed inJapanese Laid-Open Patent Applications (KOKAI) Nos. 193426/1984,193427/1984, 1985, 156047/1985, etc.

More specifically, such a ferroelectric liquid crystal device may forexample be driven by a driving embodiment as described hereinbefore withreference to FIGS. 3 to 7.

Hereinbelow, the present invention will be explained more specificallywith reference to examples. It is however to be understood that thepresent invention is not restricted to these examples.

EXAMPLE 1

A liquid crystal composition 1-A was prepared by mixing the followingcompounds in respectively indicated proportions.

    __________________________________________________________________________    Ex.    Compound    No.   Structural formula                     Wt. parts    __________________________________________________________________________    24           ##STR52##                             10    25           ##STR53##                             20    67           ##STR54##                             10           ##STR55##                             10    5           ##STR56##                             20    57           ##STR57##                             15    58           ##STR58##                             15    47           ##STR59##                             5    51           ##STR60##                             5    __________________________________________________________________________

A liquid crystal composition 1-B was prepared by mixing the followingExample compounds Nos. 1--1 and 2-4 with the above prepared composition1-A.

    __________________________________________________________________________    Ex. Comp.    No.   Structural formula              Wt. parts    __________________________________________________________________________    1-1           ##STR61##                      12    2-4           ##STR62##                      8    Composition 1-A                       80    __________________________________________________________________________

The above-prepared liquid crystal composition 1-B was used to prepare aliquid crystal device in combination with a blank cell prepared in thefollowing manner.

Two 1.1 mm-thick glass plates were provided and respectively coated withan ITO film to form an electrode for voltage application, which wasfurther coated with an insulating layer of vapor-deposited SiO₂. Theinsulating layer was further coated with a 1.0 %-solution of polyimideresin precursor (SP-510, available from Toray K.K.) indimethylacetoamide by a spinner coater rotating at 3000 rpm for 15seconds. Thereafter, the coating film was subjected to heat curing at300 ° C. for 60 min. to obtain about 120 Å-thick film. The coating filmwas rubbed with acetate fiber-planted cloth. The thus treated two glassplates were washed with isopropyl alcohol. After silica beads with anaverage particle size of 1.5 microns were dispersed on one of the glassplates, the two glass plates were applied to each other with a bondingsealing agent (Lixon Bond, available from Chisso K.K.) so that theirrubbed directions were parallel to each other and heated at 100° C. for60 min. to form a blank cell. The cell gap was found to be about 1.5microns as measured by a Berek compensator.

Then, the above-prepared liquid crystal composition 1-B was heated intoan isotropic liquid, and injected into the above prepared cell undervacuum and, after sealing, was gradually cooled at a rate of 20° C./hourto 25° C. to prepare a ferroelectric liquid crystal device.

The ferroelectric liquid crystal device was subjected to measurement ofa driving voltage margin ΔV (=V₃ -V₁) by using the driving waveforms(bias ratio =1/3) described with reference to FIGS. 4A and 4B andsetting Δt so as to provide V₁ of about 15 volts. The results are shownbelow.

    ______________________________________                10° C.                         25° C.                                   40° C.    ______________________________________    Voltage margin ΔV                  11.0 V     10.5 V    7 V    (set Δt)                  (310 μsec)                             (90 μsec)                                       (36 μsec)    ______________________________________

Further, when the temperature was changed while the voltage (V_(S)+V_(I)) was set at a central value within the voltage margin, thetemperature difference capable of driving (hereinafter called "(driving)temperature margin") was ±3.2 ° C.

Further, a contrast of 10 was attained at 25° C. during the driving.

Comparative Example 1

A liquid crystal composition 1-C was prepared by omitting Examplecompound No. 2-4 from the liquid crystal composition 1-B, i.e., byadding only Example compound No. 1--1 to the liquid crystal composition1-A, and a liquid crystal composition 1-D was prepared by omittingExample compound No. 1--1 from the composition 1-B, i.e., by adding onlyExample compound No. 2-4 to the composition 1-A.

Ferroelectric liquid crystal devices 1-A, 1-C and 1-D were prepared byusing the compositions 1-A, 1-C and 1-D, respectively, instead of thecomposition 1-B, and subjected to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    1-A      8 V         8 V         6 V             (400 μsec)                         (110 μsec)                                     (40 μsec)    1-C      8.5 V       8.0 V       6 V             (380 μsec)                         (110 μsec)                                     (40 μsec)    1-D      9.0 V       8.5 V       6 V             (350 μsec)                         (100 μsec)                                     (40 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±1.9°C. for 1-A, ±2.2° C. for 1-C and +2.4° C. for 1-D.

As apparent from the above Example 1 and Comparative Example 1, theferroelectric liquid crystal device containing the liquid crystalcomposition 1-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 2

A liquid crystal composition 2-B was prepared by mixing the followingexample compounds in the indicated proportions with the liquid crystalcomposition 1-A prepared in Example 1.

    __________________________________________________________________________    Ex. Comp.    No.   Structural formula              wt. parts    __________________________________________________________________________    1-17           ##STR63##                      16    2-43           ##STR64##                      9    Composition 1-A                       75    __________________________________________________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition 2-B wasused, and the device was subjected to measurement of driving voltagemargin and observation of switching states. In the device, a monodomainwith a good and uniform alignment characteristic was observed. Theresults of the measurement are shown below.

    ______________________________________               10° C.                         25° C.                                   40° C.    ______________________________________    Voltage margin                 11.0 V      11.0 V    7.9 V    (set Δt)                 (300 μsec)                             (80 μsec)                                       (35 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±3.4°C. A contrast of 11 was attained during the drive at the temperature.

Comparative Example 2

A liquid crystal composition 2-C was prepared by omitting Examplecompound No. 2-43 from the liquid crystal composition 2-B, i.e., byadding only Example compound No. 1-17 to the liquid crystal composition1-A, and a liquid crystal composition 2-D was prepared by omittingExample compound No. 1-17 from the composition 2-B, i.e., by adding onlyExample compound No. 2-43 to the composition 1-A.

Ferroelectric liquid crystal devices 1-A, 2-C and 2-D were prepared byusing the compositions 1-A, 2-C and 2-D, respectively, instead of thecomposition 1-B, and subjected to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    1-A      8 V         8 V         6 V             (400 μsec)                         (110 μsec)                                     (40 μsec)    2-C      8.5 V       8.5 V       6 V             (360 μsec)                         (100 μsec)                                     (40 μsec)    2-D      9.5 V       9.0 V       6 V             (300 μsec)                         (90 μsec)                                     (40 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±1.9°C. for 1-A, ±2.3° C. for 2-C and ±2.5° C. for 2-D.

As apparent from the above Example 2 and Comparative Example 2, theferroelectric liquid crystal device containing the liquid crystalcomposition 2-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 3

A liquid crystal composition 3-A was prepared by mixing the followingcompounds in respectively indicated proportions.

    __________________________________________________________________________    Ex. Comp.    No.   Structural formula                      wt. parts    __________________________________________________________________________           ##STR65##                              40    9           ##STR66##                              40    12           ##STR67##                              15    13           ##STR68##                              15    17           ##STR69##                              25    18           ##STR70##                              25    67           ##STR71##                              10    57           ##STR72##                              5    60           ##STR73##                              5    __________________________________________________________________________

A liquid crystal composition 3-B was prepared by mixing the followingexample compounds Nos. 1--1, 2-4 and 2-16 with the above preparedcomposition 3-A.

    __________________________________________________________________________    Ex. Comp.    No.   Structural formula              wt. parts    __________________________________________________________________________    1-1           ##STR74##                      10    2-4           ##STR75##                      6    2-16           ##STR76##                      4    Composition 3-A                       80    __________________________________________________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition 3-B wasused instead of the composition 1-B. The device was subjected tomeasurement of driving voltage margin and observation of switchingstates. In the device, a monodomain with a good and uniform alignmentcharacteristic was observed. The results of the measurement are shownbelow.

    ______________________________________               10° C.                         25° C.                                   40° C.    ______________________________________    Voltage margin                 12.5 V      12.0 V    8.5 V    (set Δt)                 (960 μsec)                             (290 μsec)                                       (90 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±3.4°C. A contrast of 10 was attained during the drive at the temperature.

Comparative Example 3

A liquid crystal composition 3-C was prepared by omitting Examplecompounds Nos. 2-4 and 2-16 from the liquid crystal composition 3-B,i.e., by adding only Example compound No. 1--1 to the liquid crystalcomposition 3-A, and a liquid crystal composition 3-D was prepared byomitting Example compound No. 1--1 from the composition 3-B, i.e., byadding only Example compounds Nos. 2-4 and 2-16 to the composition 3-A.

Ferroelectric liquid crystal devices 3-A, 3-C and 3-D were prepared byusing the compositions 3-A, 3-C and 3-D, respectively, instead of thecomposition 1-B, and subjected to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    3-A      10 V        10 V        8 V             (1300 μsec)                         (340 μsec)                                     (100 μsec)    3-C      11.0 V      10.5 V      8 V             (1000 μsec)                         (320 μsec)                                     (100 μsec)    3-D      12.0 V      10.5 V      8 V             (980 μsec)                         (320 μsec)                                     (100 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±2.4°C. for 3-A, ±2.6° C. for 3-C and ±2.8° C. for 3-D.

As apparent from the above Example 3 and Comparative Example 3, theferroelectric liquid crystal device containing the liquid crystalcomposition 3-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 4

A liquid crystal composition 4-B was prepared by mixing the followingexample compounds in the indicated proportions with the liquid crystalcomposition 3-A prepared in Example 3.

    __________________________________________________________________________    Ex. Comp. No.            Structural formula                wt. parts    __________________________________________________________________________    1-24             ##STR77##                        15    2-43             ##STR78##                         5    2-56             ##STR79##                         5            Composition 3-A                   75    __________________________________________________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition 4-B wasused, and the device was subjected to measurement of driving voltagemargin and observation of switching states. In the device, a monodomainwith a good and uniform alignment characteristic was observed. Theresults of the measurement are shown below.

    ______________________________________               10° C.                        25° C.                                   40° C.    ______________________________________    Voltage margin                 13.5 V     12.5 V     9.0 V    (set Δt)                 (940 μsec)                            (280 μsec)                                       (80 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±3.6°C. A contrast of 11 was attained during the drive at the temperature.

Comparative Example 4

A liquid crystal composition 4-C was prepared by omitting Examplecompounds Nos. 2-56 and 2-43 from the liquid crystal composition 4-B,i.e., by adding only Example compound No. 1-24 to the liquid crystalcomposition 3-A, and a liquid crystal composition 4-D was prepared byomitting Example compound No. 1-24 from the composition 4-B, i.e., byadding only Example compounds Nos. 2-56 and 2-43 to the composition 3-A.

Ferroelectric liquid crystal devices 3-A, 4-C and 4-D were prepared byusing the compositions 3-A, 4-C and 4-D, respectively, instead of thecomposition 1-B, and subjected to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    3-A      10 V        10 V        8 V             (1300 μsec)                         (340 μsec)                                     (100 μsec)    4-C      11.5 V      10.5 V      8 V             (1000 μsec)                         (310 μsec)                                     (100 μsec)    4-D      11.0 V      10.0 V      8 V             (1100 μsec)                         (320 μsec)                                     (100 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±2.4°C. for 3-A, ±2.8° C. for 4-C and ±2.6° C. for 4-D.

As apparent from the above Example 4 and Comparative Example 4, theferroelectric liquid crystal device containing the liquid crystalcomposition 4-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 5

A liquid crystal composition 5-A was prepared by mixing the followingcompounds in respectively indicated proportions.

    __________________________________________________________________________    Ex. Comp. No.            Structural formula                     wt. parts    __________________________________________________________________________     8             ##STR80##                             45     9             ##STR81##                             45    12             ##STR82##                             15    13             ##STR83##                             15    17             ##STR84##                             30    18             ##STR85##                             30    67             ##STR86##                             10    __________________________________________________________________________

A liquid crystal composition 5-B was prepared by mixing the followingExample compound Nos. 1-5 and 2-4 with the above prepared composition5-A.

    __________________________________________________________________________    Ex. Comp. No.            Structural formula             wt. parts    __________________________________________________________________________    1-5             ##STR87##                     12    2-4             ##STR88##                      8            Composition 5-A                80    __________________________________________________________________________

A ferroelectric liquid crystal device 5-B was prepared in the samemanner as in Example 1 except that the liquid crystal composition 5-Bwas used instead of the composition 1-B. The device was subjected tomeasurement of driving voltage margin and observation of switchingstates. In the device, a monodomain with a good and uniform alignmentcharacteristic was observed. The results of the measurement are shownbelow.

    ______________________________________               10° C.                         25° C.                                   40° C.    ______________________________________    Voltage margin                 12.0 V      12.5 V    9.0 V    (set Δt)                 (820 μsec)                             (220 μsec)                                       (72 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±3.2°C. A contrast of 10 was attained during the drive at the temperature.

Comparative Example 5

A liquid crystal composition 5-C was prepared by omitting Examplecompound No. 2-4 from the liquid crystal composition 5-B prepared inExample 5, i.e., by adding only Example compound No. 1-5 to the liquidcrystal composition 5-A, and a liquid crystal composition 5-D wasprepared by omitting Example compound No. 1-5 from the composition 5-B,i.e., by adding only Example compound No. 2-4 to the composition 5-A.

Ferroelectric liquid crystal devices 5-A, 5-C and 5-D were prepared byusing the compositions 5-A, 5-C and 5-D, respectively, instead of thecomposition 1-B, and subjected-to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    5-A      9 V         9.5 V       8.5 V             (1050 μsec)                         (280 μsec)                                     (78 μsec)    5-C      10.0 V      10.0 V      8.5 V             (920 μsec)                         (260 μsec)                                     (78 μsec)    5-D      10.5 V      10.0 V      8.5 V             (900 μsec)                         (260 μsec)                                     (78 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±2.2°C. for 5-A, ±2.3° C. for 5-C and ±2.5° C. for 5-D.

As apparent from the above Example 5 and Comparative Example 5, theferroelectric liquid crystal device containing the liquid crystalcomposition 5-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 6

A liquid crystal composition 6-B was prepared by mixing the followingexample compounds in the indicated proportions with the liquid crystalcomposition 5-A prepared in Example 5.

    __________________________________________________________________________    Ex. Comp. No.            Structural formula              wt. parts    __________________________________________________________________________    1-3             ##STR89##                      16    2-43             ##STR90##                       9            Composition 5-A                 75    __________________________________________________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition 6-B wasused, and the device was subjected to measurement of driving voltagemargin and observation of switching states. In the device, a monodomainwith a good and uniform alignment characteristic was observed. Theresults of the measurement are shown below.

    ______________________________________               10° C.                         25° C.                                   40° C.    ______________________________________    Voltage margin                 12.0 V      13.0 V    9.0 V    (set Δt)                 (800 μsec)                             (210 μsec)                                       (72 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±3.6°C. A contrast of 11 was attained during the drive at the temperature.

Comparative Example 6

A liquid crystal composition 6-C was prepared by omitting Examplecompound No. 2-43 from the liquid crystal composition 6-B, i.e., byadding only Example compound No. 1-3 to the liquid crystal composition5-A, and a liquid crystal composition 6-D was prepared by omittingExample compound No. 1-3 from the composition 6-B, i.e., by adding onlyExample compound No. 2-43 to the composition 5-A.

Ferroelectric liquid crystal devices 5-A, 6-C and 6-D were prepared byusing the compositions 5-A, 6-C and 6-D, respectively, instead of thecomposition 1-B, and subjected to measurement of driving voltage marginΔV, otherwise in the same manner as in Example 1. The results are shownbelow.

    ______________________________________    Voltage margin ΔV (set Δt)           10° C.                     25° C.                                 40° C.    ______________________________________    5-A      9 V         9.5 V       8.5 V             (1050 μsec)                         (280 μsec)                                     (78 μsec)    6-C      9.5 V       10.0 V      8.5 V             (960 μsec)                         (260 μsec)                                     (78 μsec)    6-D      10.5 V      10.5 V      8.5 V             (900 μsec)                         (240 μsec)                                     (78 μsec)    ______________________________________

Further, the driving temperature margin with respect to 25° C. was ±2.2°C. for 5-A, ±2.4° C. for 6-C and ±2.8° C. for 6-D.

As apparent from the above Example 6 and Comparative Example 6, theferroelectric liquid crystal device containing the liquid crystalcomposition 6-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap.

EXAMPLE 7

A blank cell was prepared in the same manner as in Example 1 except foromitting the SiO₂ layer to form an alignment control layer composed ofthe polyimide resin layer alone on each electrode plate. Fourferroelectric liquid crystal devices were prepared by filling such ablank cell with liquid crystal compositions 1-B, 1-C, 1-D and 1-A,respectively, prepared in Example 1 and Comparative Example 1. Theseliquid crystal devices were subjected to measurement of driving voltageand temperature margins in the same manner as in Example 1. The resultsare shown below.

    ______________________________________    Voltage margin (set Δt)                             Temp. margin    Comp.  10° C.                     25° C.                                40° C.                                       (at 25° C.)    ______________________________________    1-B    12.0 V    11.0 V     8.0 V  ±3.2° C.           (260 μsec)                     (82 μsec)                                (40 μsec)    1-C    9.5 V     8.5 V      7.0 V  ±2.2° C.           (360 μsec)                     (100 μsec)                                (40 μsec)    1-D    10.0 V    9.0 V      6.5 V  ±2.4° C.           (320 μsec)                     (90 μsec)                                (40 μsec)    1-A    8.2 V     8.3 V      6.3 V  ±2.0° C.           (370 μsec)                     (100 μsec)                                (40 μsec)    ______________________________________

As is apparent from the above Example 7, also in the case of a differentdevice structure, the device containing the ferroelectric liquid crystalcomposition 2-B according to the present invention provided widerdriving voltage and temperature margins and showed a better performanceof retaining good images in resistance to changes in environmentaltemperature and cell gap than the device containing the other liquidcrystal compositions.

EXAMPLE 8-15

Liquid crystal compositions 8-B to 15-B were prepared by replacing theexample compounds and the liquid crystal compositions used in Example 1,3 and 5 with example compounds and liquid crystal compositions shown inthe following Table 1. Ferroelectric liquid crystal devices wereprepared by respectively using these compositions instead of thecomposition 1-B, and subjected to measurement of driving margins andobservation of switching states. In the devices, a monodomain with agood and uniform alignment characteristic was observed. The results ofthe measurement are shown in the following Table 1.

                                      TABLE 1    __________________________________________________________________________           Example compound No. Voltage margin    Ex. No.           or liquid crystal composition No.                                (V)      Temp.    (Comp. No.)           (weight parts)       Set Δt (μsec)                                         margin (°C.)    __________________________________________________________________________    8      1-1              1-6                 1-36                    2-4                       2-14                          2-20                             1-A                                9.8      ±3.0    (8-B)  (5)              (3)                 (4)                    (2)                       (3)                          (3)                             (80)                                90    9      1-2              1-8                 1-25                    2-43                       2-46                          2-48                             1-A                                9.6      ±3.0    (9-B)  (5)              (4)                 (3)                    (2)                       (2)                          (4)                             (80)                                90    10     1-4              1-13                 1-29                    2-7                       2-34                          2-16                             3-A                                12.8     ±3.4    (10-B) (4)              (4)                 (8)                    (3)                       (3)                          (3)                             (75)                                285    11     1-5              1-14                 1-50                    2-45                       2-46                          2-56                             3-A                                12.2     ±3.2    (11-B) (4)              (6)                 (6)                    (3)                       (3)                          (3)                             (75)                                270    12     1-7              1-9                 1-39                    2-16                       2-39                          2-40                             5-A                                11.8     ±3.3    (12-B) (5)              (3)                 (4)                    (2)                       (3)                          (3)                             (80)                                210    13     1-6              1-10                 1-12                    2-46                       2-50                          2-59                             5-A                                11.6     ±3.1    (13-B) (5)              (4)                 (3)                    (4)                       (2)                          (2)                             (80)                                200    14     1-8              1-14                 1-40                    2-25                       2-32                          2-33                             1-A                                10.5     ±3.1    (14-13)           (4)              (4)                 (8)                    (2)                       (2)                          (4)                             (75)                                90    15     1-17              1-20                 1-47                    2-51                       2-60                          2-66                             3-A                                12.0     ±3.4    (15-B) (5)              (3)                 (4)                    (2)                       (2)                          (5)                             (80)                                285    __________________________________________________________________________

As is apparent from the results shown in the above Table 1, theferroelectric liquid crystal devices containing the liquid crystalcompositions 8-B to 15-B provided wide driving voltage and temperaturemargins and showed good performances of retaining good images inresistance to changes in environmental temperature and cell gap.

EXAMPLE 16

A liquid crystal composition 16-B was prepared by mixing the followingexample compound in the indicated proportion with the liquid crystalcomposition 1-B prepared in Example 1.

    ______________________________________    Ex.                               wt.    Comp. No.            Structural formula        parts    ______________________________________    3-10             ##STR91##                10            Composition 1-B           90    ______________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition was used,and the device was subjected to measurement of driving voltage margin inthe same manner as in Example 1 to obtain the following results.

    ______________________________________               10° C.                        25° C.                                  40° C.    ______________________________________    Voltage margin                 10.0 V     10.5 V    7.0 V    (set Δt)                 (370 μsec)                            (95 μsec)                                      (38 μsec)    ______________________________________

Then, the tilt angle of the above device was measured under right-anglecross nicols at 25° C. to provide 7.8 degrees. Further, the tilt angleof the device was again measured while being subjected to application ofrectangular waveforms of ±8 V and a frequency of 60 KHz and found to be13.5 degrees. The transmittance measured at that time was 14.0%, and acontrast of 70:1 was attained.

Comparative Example 16

A liquid crystal composition 16-C was prepared in the same manner as inExample 16 except that the liquid crystal composition 1-A prepared inExample 1 was used instead of the composition 1-B to be mixed with theExample compound No. 3-10 in the same proportions.

Ferroelectric liquid crystal devices were prepared by using thecompositions 16-C, 1-A and 1-B respectively and subjected to measurementof driving voltage margin, otherwise in the same manner as in Example 1.Further, the tilt angles of these devices were measured in the samemanner as in Example 16. The results are shown below.

    ______________________________________    Voltage margin (set Δt)    Comp.     10° C.                          25° C.                                      40° C.    ______________________________________    1-A       8.0 V       8.0 V       6.0 V              (400 μsec)                          (110 μsec)                                      (40 μsec)    1-B       11.0 v      10.5 V      7.0 V              (310 μsec)                          (90 μsec)                                      (36 μsec)    16-C      7.0 V       7.5 V       6.0 V              (450 μsec)                          (120 μsec)                                      (40 μsec)    ______________________________________    Tilt angle (25° C.)            Initial      Under AC appln.    Comp.   (no electric field)                         (60 KHz, ±8 V, rectangular)    ______________________________________    1-A     7.8    degrees   8.0       degrees    1-B     7.7    degrees   7.8       degrees    16-C    8.0    degrees   13.7      degrees    ______________________________________

As apparent from Example 16 and Comparative Example 16, the liquidcrystal composition 16-B obtained by mixing a mesomorphic compoundhaving a negative dielectric anisotropy (Example compound No. 3-10) withthe liquid crystal composition 1-B according to the present inventionprovided a wider driving margin and also provided a remarkably improveddisplay characteristic when used in a display method utilizing ACapplication (or AC stabilization).

EXAMPLE 17

A liquid crystal composition 17-B was prepared by mixing the followingexample compounds in the respectively indicated proportions with theliquid crystal composition 1-B prepared in Example 1.

    __________________________________________________________________________    Ex. Comp. No.            Structural formula               wt. parts    __________________________________________________________________________    3-90             ##STR92##                       5    3-12             ##STR93##                       5    3-122             ##STR94##                       2    3-70             ##STR95##                       3    3-107             ##STR96##                       3    3-111             ##STR97##                       1    3-167             ##STR98##                       1            Composition 1-B                  80    __________________________________________________________________________

A ferroelectric liquid crystal device was prepared in the same manner asin Example 1 except that the above liquid crystal composition was used,and the device was subjected to measurement of driving voltage margin inthe same manner as in Example 1 to obtain the following results.

    ______________________________________               10° C.                        25° C.                                  40° C.    ______________________________________    Voltage margin                 11.0 V     10.0 V    7.5 V    (set Δt)                 (330 μsec)                            (95 μsec)                                      (38 μsec)    ______________________________________

Then, the tilt angle of the above device was measured under right-anglecross nicols at 25° C. to provide 8.7 degrees. Further, the tilt angleof the device was again measured while being subjected to application ofrectangular waveforms of ±8 V and a frequency of 60 KHz and found to be13.3 degrees. The transmittance measured at that time was 13.6%, and acontrast of 59:1 was attained.

Comparative Example 17

A liquid crystal composition 17-C was prepared in the same manner as inExample 17 except that the liquid crystal composition 1-A prepared inExample 1 was used instead of the composition 1-B to be mixed with theother example compounds in the same proportions.

Ferroelectric liquid crystal devices were prepared by using thecompositions 17-C, 1-A and 1-B respectively and subjected to measurementof driving voltage margin, otherwise in the same manner as in Example 1.Further, the tilt angles of these devices were measured in the samemanner as in Example 16. The results are shown below.

    ______________________________________    Voltage margin (set Δt)    Comp.     10° C.                          25° C.                                      40° C.    ______________________________________    1-A       8.0 V       8.0 V       6.0 V              (400 μsec)                          (110 μsec)                                      (40 μsec)    1-B       11.0 V      10.5 V      7.0 V              (310 μsec)                          (90 μsec)                                      (36 μsec)    17-C      7.5 V       7.5 V       6.5 V              (410 μsec)                          (120 μsec)                                      (45 μsec)    ______________________________________    Tilt angle (25° C.)            Initial      Under AC appln.    Comp.   (no electric field)                         (60 KHz, ±8 V, rectangular)    ______________________________________    1-A     7.8    degrees   8.0       degrees    1-B     7.7    degrees   7.8       degrees    17-C    9.0    degrees   13.7      degrees    ______________________________________

As apparent from Example 17 and Comparative Example 17, the liquidcrystal composition 17-B obtained by mixing mesomorphic compounds havinga negative dielectric anisotropy with the liquid crystal composition 1-Baccording to the present invention provided a wider driving margin andalso provided a remarkably improved display characteristic when used ina display method utilizing AC application (or AC stabilization).

For example, the dielectric anisotropy Δε of a mesomorphic compound or aliquid crystal composition referred to herein may be measured in thefollowing manner.

A 5 micron-thick homogeneous alignment cell having an electrode of 0.7cm² in area and a homogeneous alignment layer (rubbed polyimide) on bothsubstrates, and a 5 micron-thick homeotropic alignment cell having anelectrode of 0.7 cm² in area and a homeotropic alignment layer (aligningagent: "ODS-E" available from Chisso K.K.) on both substrates, areprovided. The respective cells are filled with a sample liquid crystalmaterial (compound or composition) to prepare liquid crystal devices.The capacitances of the liquid crystal layers are measured by applying asine wave with a frequency of 100 KHz and amplitudes of ±0.5 V to therespective devices at a prescribed temperature set for the liquidcrystal material, and the dielectric constants ε_(//) and ε.sub.⊥ areobtained from the measured capacitance values of the respective devices,whereby the dielectric anisotropy Δε is calculated by the equation ofΔε=ε_(//) -ε.sub.⊥.

As described hereinabove, the ferroelectric liquid crystal compositionaccording to the present invention provides a liquid crystal devicewhich shows a good switching characteristic, a wide driving voltagemargin and a wide temperature margin so that the device shows anexcellent performance of retaining good images in resistance to changesin environmental temperature and cell gap. Further, the liquid crystalcomposition according to the present invention further containing amesomorphic compound having a negative dielectric anisotropy, provides aliquid crystal device which retains the above-mentioned characteristicsand further shows a remarkably improved display characteristic when usedin a driving method utilizing AC stabilization.

What is claimed is:
 1. A ferroelectric chiral smectic liquid crystalcomposition comprising:at least one compound represented by thefollowing formula (I): ##STR99## wherein R₁ denotes a linear or branchedalkyl group having 1-18 carbon atoms which is optionally substitutedwith alkoxy group or halogen, R₂ denotes a linear or branched alkylgroup having 1-12 carbon atoms; X₁ denotes a single bond, ##STR100## atleast one compound represented by the following formula (II): ##STR101##wherein R₃ and R₄ denote a linear or branched alkyl group having 1-18carbon atoms, at least one of R₃ and R₄ being optically active; and X₂and X₃ denote a single bond, ##STR102## wherein said compositioncontains at least one mesomorphic compound having a dielectric anistropyΔε of below -2.
 2. A composition according to claim 1, wherein saidmesomorphic compound has a dielectric anisotropy Δε of below -5.
 3. Acomposition according to claim 2, wherein said mesomorphic compound hasa dielectric anisotropy Δε of below -10.
 4. A ferroelectric chiralsmectic liquid crystal composition comprising:at least one compoundrepresented by the following formula (I): ##STR103## wherein R₁ denotesa linear or branched alkyl group having 1-18 carbon atoms which isoptionally substituted with alkoxy group or halogen, R₂ denotes a linearor branched alkyl group having 1-12 carbon atoms; X₁ denotes a singlebond ##STR104## at least one compound represented by the followingformula (II): ##STR105## wherein R₃ and R₄ denote a linear or branchedalkyl group having 1-18 carbon atoms, at least one of R₃ and R₄ beingoptically active; and X₂ and X₃ denote a single bond, ##STR106## and amesomorphic compound having a negative dielectric anisotropy representedby any of the following Formulae (III-1) to (III-5); ##STR107## whereinRa and Rb denote a linear or branched alkyl group wherein Rb isoptionally substituted with alkoxy group; Xa and Xd denote a singlebond, ##STR108## Xb and Xc denote a single bond, ##STR109## or --CH₂ CH₂--; Aa and Ab denote a single bond ##STR110## with proviso that when Aaand Ab are both single bonds, Xb and Xc are both single bonds, and Xaand Xd are both single bonds or --O--, or Xa is ##STR111## and Xd is##STR112## and Ya and Yb are cyano group, halogen or hydrogen withproviso that Ya and Yb cannot be hydrogen simultaneously; ##STR113##wherein Re and Rf denote a linear or branched alkyl group; Xe and Xh area single bond, ##STR114## Xf and Xg are ##STR115## or a single bond; andAe and Af are ##STR116## or a single bond with proviso that Ae and Afcannot be a single bond simultaneously; ##STR117## wherein Ai is asingle bond or ##STR118## Aj is a single bond, ##STR119## Ri and Rj area linear or branched alkyl group wherein Ri is optionally substitutedwith Cl radical and Rj is optionally substituted with alkoxy group withproviso that Ri and Rj are linear alkyl groups when Aj is a single bond;Z₁ is --O-- or --S--; Xi and Xk are a single bond, ##STR120## Xj is asingle bond, ##STR121## --CH₂ O-- or --OCH₂ -- with proviso that Xi is asingle bond when Ai is a single bond, Xj is not a single bond when Aj is##STR122## and Xk is a single bond when Aj is a single bond; ##STR123##wherein Rl and Rm are a linear or branched alkyl group; Al and Am are asingle bond, ##STR124## with proviso that Al and Am cannot be a singlebond simultaneously; Xl is a single bond, ##STR125## and Xm is a singlebond, ##STR126## ##STR127## wherein Rn and Ro are a linear or branchedalkyl group; Xn single bond, ##STR128## Xo and Xp are a single bond,##STR129## --CH₂ O--, --OCH₂ -- or --CH₂ CH₂ --; An and Ap are a singlebond, ##STR130## Ao is ##STR131## and Z₂ is ##STR132##
 5. A liquidcrystal device, comprising a pair or electrode plates and aferroelectric liquid crystal composition according to any one of claims1-4 disposed between the electrode plates.